1. Field of the Invention
The present invention relates to a transmission system, transmission method, transmission apparatus, and electronic recording medium having recorded therein a transmission program, which are configured to transmit a plurality of signals using a plurality of carriers between the transmitter and the receiver, for example, a multicarrier/DS-CDMA.
2. Description of the Related Art
The wideband code division multiple access (W-CDMA) and the cdma2000 have been adopted as the method of a radio access for the 3rd generation mobile telecommunications system (IMT-2000). The radio bandwidth per carrier is 5 MHz for the W-CDMA, and is 1.25 MHz for the cdma2000.
In the mobile communication environment, a radio signal transmitted reaches the receiver as a plurality of radio signals (multipath signal) of which each signal has a different propagation delay due to the effect of buildings and over-ground objects, etc. located between the transmitter and the receiver, called “multipath propagation”.
With respect to the multipath signal, the direct sequence code division multiple access (DS-CDMA), which is the basis of the W-CDMA and the cdma2000, resolves a plurality of the multipath signals into independent signals and aligns the phase of the independent signals so as to combine them into one composite signal (it is called RAKE reception). As the result of the combining the signals, desired signal power of the radio signal can be increased, namely RAKE time-diversity effect, and the DS-CDMA can therefore improve receiving performance.
Especially, since the W-CDMA has the radio bandwidth of 5 MHz, the minimum time interval for resolving multipath signal is approximately one fourth of the cdma2000 whose radio bandwidth is 1.25 MHz. Therefore, in the W-CDMA, a received signal can be resolved into a larger number of paths and thereby perform the RAKE reception effectively. The results of experimental examination with regard to the improvement of receiving performance, which is achieved by expanding the radio bandwidth with the RAKE time-diversity effect, have been reported in the literature “Further Results on Field Experiments of Coherent Wideband DS-CDMA Mobile Radio (T. Dohi, Y. Okumura, and F. Adachi: IEICE Trans. Commun., Vol. E81-B, No.6, pp. 1239–1247, June, 1998)”.
This literature shows the results of a field experiment for transmission of the DS-CDMA radio signal using the radio bandwidth from 1.25 MHz to 10 MHz. In this literature, it has been reported that the Rake reception is possible with a larger number of resolved paths by expanding the radio bandwidth up to 10 MHz. It has also been reported that, as a result of the above, the receiving performance of the radio signal can be improved by the RAKE time-diversity effect.
Further, in the literature “Performance of Rake Reception in Dense Multipath Channels: Implications of Spreading Bandwidth and Selection Diversity Order (M. Z. Win, G. Chrisikos, and N. R. Sollenberger: IEEE JSAC.c vol.18, No.8, pp, 1516–1525, August, 2000)”, it has been analyzed that by expanding the radio bandwidth, the number of the multipath path increases and the receiving performance of the radio signal of the DS-CDMA is improved if RAKE reception is performed using these increased multipath signals. Moreover, this literature shows that the RAKE time-diversity effect decreases as the number of multipath increases, and a level of the improvement of the receiving performance achieved by the RAKE time-diversity effect becomes saturated. Incidentally, in this literature, since it has been assumed that a timing of the respective resolved multipath signals is known, consideration to the accuracy of path timing detection have not been given.
However, in an actual environment, in a case where large numbers of the multipath signals are observed due to expansion of the radio bandwidth, a multipath interference (hereinafter referred as MPI) which the respective signals interfere each other at the receiving unit occurs. Due to the MPI, signal power per path decreases and it causes that the accuracy of the path timing detection for respective paths becomes deteriorated. Moreover, the accuracy of channel estimation, which estimates volume of a fluctuation occurred on the radio link based to the detected signal, becomes deteriorated accordingly and it affects the receiving performance of the radio signal at the receiver.
Considering the above situation, in the literature “Experimental Evaluation of Coherent Rake Receiver for Broadband DS-CDMA Mobile Radio (T. Ikeda, K. Okaga, M. Sawahashi, and F. Adachi: IEEE VTC Spring, pp. 1849–1853, 2000)”, the evaluation has been made for the receiving performance under the RAKE reception taking into account of the accuracy of the path timing detection as well as the channel estimation by conducting a laboratory experiment for transmission of the DS-CDMA radio signal using a 100 MHz bandwidth. According to the experimental result, it has been stated that in a case where the multipath exists in very large number, a level of the RAKE time-diversity effect tends to get saturated or decreases due to the MPI and the deterioration of the accuracy of the path timing detection as well as the accuracy of the channel estimation.
Accordingly, as for the DS-CDMA transmission, it is considered that an appropriate radio bandwidth can be determined based on the relationship between the improvement of the receiving performance of the radio signal, which is achieved by the RAKE time-diversity effect, and the deterioration of the receiving performance due to increase in the MPI as well as decrease in a signal power per path.
Conventionally, as for considerations to a mobile communications system, approximately several megahertz (MHz) of bandwidth was adopted even at maximum.
However, it has been expected that a transmission rate of more than several tens of Mbps be achieved even in the mobile communications system. It is therefore anticipated that the radio bandwidth to be assigned for the mobile communications system, will be expanded and it is highly possible that several tens to 100 MHz, or more, of radio bandwidth be assigned for the system. In a case where coping with the expansion of the radio bandwidth by using the multicarrier/DS-CDMA as the radio access, there is the problem that the effect on the radio link, specifically the problem that due to the MPI, the receiving performance of the radio signal greatly varies depending on radio bandwidth per carrier, i.e., the number of carriers.